Perinatal deficiency of thyroid hormone (TH) or hypothyroidism results in abnormal development of the central nervous system, typified by poor neurite growth and synaptogenesis in the cerebellum. Brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3), members of the neurotrophic factor family, may serve critical roles in this process since NT-3 and BDNF levels are stimulated by TH, and TH treatment, or replacement of NT-3 or BDNF in part, reverses the abnormal nerve growth and development in the cerebellum. Further, a mutant mouse, staggerer (sg), in which the orphan nuclear hormone receptor RORalpha gene is disrupted, exhibits morphological and neurological abnormalities of the cerebellum similar to those seen in hypothyroid animals. In this mouse, TH does not alter the abnormal phenotype. Cerebellar Purkinje cells in sg-wild type mouse chimeras lack aberrant morphologies suggesting that trophic factors secreted from wild type cells can rescue sg cells. These results indicate that RORalpha may be involved in the TH-regulated expression of neurotrophic factors and other genes. The main goal of this proposal is to determine the mechanisms by which TH regulates neurotrophin gene expression in the cerebellum, including the role of RORalpha. The specific aims are: (1) to determine the effect of thyroid status on the developmental changes in BDNF, NT-3, Pcp-2, calbindin and cytochrome c oxidase I gene expression in the cerebellum of normal and sg mice in vivo; (2) to examine the effects of TH on BDNF, NT-3 and RORalpha gene expression, and TH, BDNF and NT-3 on neuropil outgrowth and synaptogenesis in primary cultures of cerebellum of normal and sg mice in vitro; (3) to examine the direct involvement of TR and RORalpha in neurotrophin and RORalpha (for TR) gene expression and cerebellar Purkinje cell neurogenesis, using transgenic mice expressing dominant-negative TR and RORalpha in Purkinje cells, respectively; (4) to investigate the potential involvement of RORalpha in TR action on BDNF, NT-3 and other cerebellar genes that are directly regulated by TR, using promoter analyses, transfection studies and DNA binding assays; (5) to identify and characterize other cerebellar genes that play critical roles in cerebellar neurogenesis and are regulated by TH, using differential hybridization and PCR-based subtraction techniques. These studies will provide new insights into the role of TH in brain development, which may have clinical importance in cretinism and infantile hypothyroidism.